The present invention relates to pattern defect inspecting apparatus and more particularly, to a pattern defect inspecting apparatus which can suitably inspect a defect in a pattern in fabrication of semiconductor devices, image pick-up elements, display elements, etc.
Major examples of pattern defect inspecting apparatuses associated with the present invention include a scanning electron microscope (SEM), a laser scanning microscope, and a scanning inter-atomic force microscope. Explanation will be made in connection with an example of semiconductor fabrication as a typical application field. The SEM has been widely used in inspection of pattern defect. Thus explanation will be made in connection with the use of the SEM.
FIG. 1 is an arrangement of an SEM for explaining its basic principle, in which an electron beam is used as a probe to perform a raster scan on the entire surface of a field of view.
An electron beam 2 emitted from an electron gun 1 is accelerated, and converged through a condenser lens 3 and an objective lens 4, and then focused on a surface of a wafer 5 as an sample. Concurrently, the electron beam 2 is bent in its locus by a deflector 6 so that the beam two-dimensionally or one-dimensionally scans the entire surface of the wafer. Meanwhile, a part of the wafer, when subjected to an irradiation of the electron beam 2, emits secondary electrons. The secondary electrons are detected by a secondary electron detector 8 to be converted to an electric signal, the signal is converted by an A/D converter 9 to a digital signal and stored in an image memory 10. The stored signal is processed by an image processor 11 to be used for brightness modulation or Y modulation of a display unit 12. The display unit 12 is scanned similarly to the scanning of the electron beam 2 on the wafer, so that a sample image is formed on the display unit 12. When two-dimensional scanning and brightness modulation are carried out, an image appears on the display unit; whereas, when the Y modulation is carried out, a line profile is depicted on the display.
Here is an example of procedure of inspecting a pattern defect with use of the SEM.
A sheet of wafer 5 to be measured is extracted from a wafer cassette 13, and then subjected to a pre-aligning operation. The pre-alignment is to align the wafer direction with respect to an orientation flat or notch formed in the wafer as a reference. More in detail, after being subjected to the pre-alignment, the wafer 5 is fed into a sample chamber 14 kept in a vacuum and then placed on an X-Y stage 15 in the chamber. The wafer 5 placed on the X-Y stage 15 is aligned with use of an optical microscope 16 mounted in an upper part of the sample chamber 14. The alignment in this example is to correct a relationship between a positional coordinate system of the X-Y stage 15 and a pattern positional coordinate system of the wafer, for which end an alignment pattern formed on the wafer is used. More specifically, an image generated by the optical microscope 16 is converted by a CCD element or the like to an electric signal, converted by an A/D converter 17 to a digital signal, and then stored in the image memory 10. The stored signal is coupled to the display unit 12 via the image processor 11 so that the image of the optical microscope appears on the display unit 12. The image of the optical microscope magnified to a size about several hundred times the size of the alignment pattern is compared with a reference image of an alignment pattern previously registered, and the stage positional coordinate system is corrected so that its field of view exactly overlaps with the field of view of the reference image. After the alignment, a raster scan is carried out on the entire surface of a required inspection zone on the wafer using combination of the electron beam scan and stage movement to thereby form an SEM image. The formed SEM image is compared with the reference SEM image so that a difference between the images is detected as a pattern defect. Generally used as the reference SEM image is an SEM image of the same part in a chip or cell already inspected.
In this connection, control over the storing and reading operation of the image signal, the processing of the image signal, pattern matching, etc. is carried out under control of a computer/controller 18.
There is a reciprocal relationship between detection sensitivity for pattern defects and an inspection rate therefor. Both the detection sensitivity and inspection rate depend on pixel size (the number of pixels in the view field). When the pixel size is made small (when the number of pixels in the view field is increased), the pattern defect detection sensitivity can be made high but the inspection rate is decreased. That is, when a pattern defect is to be detected with a high sensitivity, its required inspection time is increased.
There is also a correlation between the pattern defect detection sensitivity and error detection frequency. The defect detection sensitivity is proportional to the resolution of the SEM. When the SEM resolution is increased in order to increase the defect detection sensitivity, a fine structure other than the pattern edge becomes also clear. The clear fine structure is frequently erroneously judged as a pattern edge, which leads to noise caused by the pattern defect (error detection factor). In other words, when the pattern defect is detected with a high sensitivity, the error detection rate is increased.